1. Field of the Invention
The present invention relates to an electric power generation method using a thermoelectric power generation element, which is a method of obtaining electrical energy directly from thermal energy. Furthermore, the present invention also relates to a thermoelectric power generation element that converts thermal energy directly into electrical energy and the method of producing the same, as well as a thermoelectric power generation device.
2. Description of the Related Art
Thermoelectric power generation is a technique for converting thermal energy directly into electrical energy by utilizing the Seebeck effect whereby an electromotive force is generated in proportion to the temperature difference applied to both ends of a material. This technique is used practically in, for example, remote area power supply, space power supply, and military power supply.
A conventional thermoelectric power generation element generally has a structure that is referred to as a so-called “π-type structure”, in which a “p-type semiconductor” and an “n-type semiconductor” that are different in carrier sign from each other are combined together thermally in parallel and electrically in series.
Generally, the performance of a thermoelectric material that is used for a thermoelectric power generation element is evaluated by a figure of merit Z or a figure of merit ZT nondimensionalized by multiplying Z by absolute temperature. ZT can be expressed by a formula of ZT=S2/ρκ, where S denotes the Seebeck coefficient of a thermoelectric material, ρ indicates electrical resistivity, and κ is thermal conductivity. Furthermore, S2/ρ, which is an index expressed with consideration being given to only the Seebeck coefficient S and electrical resistivity ρ, also is referred to as a power factor (output factor) and serves as a criterion for evaluating the power generation performance of a thermoelectric material obtained when the temperature difference is constant.
Bi2Te3 that currently is used practically as a thermoelectric material has a high thermoelectric power generation performance, specifically, a ZT of approximately 1 and a power factor of about 40 μW/(cm·K2). However, in the case of an element having the aforementioned π-type structure, it is difficult to obtain a high thermoelectric power generation performance. Thus, it has not reached a level that is sufficiently high enough to allow it to be used practically for more various applications. Furthermore, Bi2Te3 has a problem in heat resistance and therefore the thermoelectric power generation performance thereof deteriorates at temperatures of 100° C. or higher, and further Bi2Te3 poses a heavy load on the environment since it contains Bi as a constituent element.
In JP 2004-186241 A (Reference 1), boron compounds such as SrB6 are examined as materials for thermoelectric power generation. These boron compounds are chemically very stable under normal temperature and pressure and are stable at temperatures up to around 1000K in the air and up to around 2500K under an atmosphere of an inert gas such as nitrogen. Thus they have excellent heat resistance. Furthermore, the aforementioned compounds are free from constituent elements that pose a heavy load on the environment. However, the boron compounds have a power factor of about 18 μW/(cm·K2). When an element having the above-mentioned π-type structure is formed using one of them, the thermoelectric power generation performance actually obtained from the element further deteriorates, which prevents them from being used practically.
On the other hand, there has been a proposal for an element utilizing anisotropy of the thermoelectric properties in a laminated structure that is naturally-occurring or is produced artificially as an element having a different structure from the π-type structure (Thermoelectrics Handbook, Chapter 45 “Anisotropic Thermoelements”, CRC Press (2006): Reference 2). However, according to Reference 2, it is difficult to improve ZT of such an element. Therefore, it is developed not for the use for thermoelectric power generation but for assumed uses mainly in the field of measurement such as an infrared sensor.
Furthermore, JP 6(1994)-310766 A (Reference 3) discloses, as a thermoelectric material having a similar structure thereto, a material in which a material having thermoelectric properties, which is typified by Fe—Si materials, and an insulating material with a thickness of 100 nm or less, which is typified by SiO2, are arranged alternately in the form of stripes on a substrate. According to Reference 3, in a material having such a microstructure, as compared to the case where a Fe—Si material having thermoelectric properties is used independently, the Seebeck coefficient S can be improved but on the other hand, the electrical resistivity ρ increases due to the insulating material contained therein. Accordingly, the element made thereof has increased internal resistance, and the electric power obtained therewith is reduced conversely.
Examples of other thermoelectric materials having laminated structures include a material having a layered body formed of semimetal, metal, or synthetic resin, which is disclosed in WO 00/076006 (Reference 4). This material is based on the configuration in which, as in the case of the conventional π-type structure, a temperature difference is applied to the direction in which the respective layers of the layered body are laminated, and thereby electric power is extracted through a pair of electrodes that are disposed so as to oppose in the same direction as that described above. Therefore the element disclosed in Reference 4 is substantially different from that disclosed in Reference 1.